Ephemeral Regions Research Articles

A converging flux model for the formation of an X-ray bright point above a supergranule cell

Abstract The many complex regions of positive and negative flux that thread the surface of the Sun are mainly grouped around the edges of supergranule cells. These cells have large concentrations of magnetic flux on their boundaries and very little flux inside, with the magnetic fragments that appear in the centre of the cells swept to the boundaries by convective motions. Thus, a small bipolar pair of magnetic fragments (such as an ephemeral region) emerges inside a cell and moves towards the cell boundary as it grows. On reaching the boundary the fragments encounter unipolar regions of network flux with which they may merge or cancel. When cancellation takes place there is often an associated X-ray bright point in the overlying corona. Here, the emergence and interaction of an ephemeral region in a quiet-region or active-region super-granule cell is considered. It is found that there are three possible scenarios for the evolution of an ephemeral region in a supergranule cell and these are all investigated. The magnetic fields for the supergranule cell and ephemeral region are modelled by finite sources of flux and are studied as the ephemeral region moves through a series of quasi-static states. It is found that the ratio of the cancelling fragment widths (strengths) is important in determining the lifetime and path of the bright point, while the actual sizes of the fragments is important for determining the intensity of the bright point, the lifetime of the cancelling magnetic feature and the relative times of the bright point completion and cancelling magnetic feature onset. From this we suggest that transient brightenings in active regions and bright points on the quiet Sun may both be created by the converging flux mechanism detailed here.

Characteristics of Small-Scale Magnetic Fields in a Solar Coronal Hole

AbstractThe photospheric longitudinal magnetograms of a coronal hole are used to study the small-scale magnetic fields in the coronal hole. The research is concentrated on the characteristics of the network and the ephemeral region fields, and the results are compared with those in the ordinary quiet regions with mixed magnetic polarities.

The correspondence between x-ray bright points and evolving magnetic features in the quiet sun

Coronal bright points, first identified as X-ray Bright Points (XBPs), are compact, short-lived and associated with small-scale, opposite polarity magnetic flux features. Previous studies have yielded contradictory results suggesting that XBPs are either primarily a signature of emerging flux in the quiet Sun, or of the disappearance of pre-existing flux. With the goal of improving our understanding of the evolution of the quiet Sun magnetic field, we present results of a study of more recent data on XBPs and small-scale evolving magnetic structures. The coordinated data set consists of X-ray images obtained during rocket flights on 15 August and 11 December, 1987, full-disk magnetograms obtained at the National Solar Observatory - Kitt Peak, and time-lapse magnetograms of multiple fields obtained at Big Bear Solar Observatory. We find that XBPs were more frequently associated with pre-existing magnetic features of opposite polarity which appeared to be cancelling than with emerging or new flux regions. Most young, emerging regions were not associated with XBPs. However, some XBPs were associated with older ephemeral regions, some of which were cancelling with existing network or intranetwork poles. Nearly all of the XBPs corresponded to opposite polarity magnetic features which wereconverging towards each other; some of these had not yet begun cancelling. We suggest that most XBPs form when converging flow brings oppositely directed field lines together, leading to reconnection and heating of the newly-formed loops in the low corona.

Properties of Emerging Bipolar Active Regions

AbstractCharacteristics and the emergence pattern of bipolar active regions are investigated using NSO/KP full disk magnetograms. Discussed in this paper are two samples of newly emerged bipolar regions: 9492 ephemeral regions and 978 active regions. Their distributions in size, latitude, and orientation are compared. It is concluded that ephemeral regions are the small scale end of a wide spectrum of active regions.

Small-Scale Magnetic Features Observed in the Photosphere

Small-scale solar features identifiable on the quiet sun in magnetograms of the line-of-sight component consist of network, intranetwork, ephemeral region magnetic fields, and the elementary bipoles of ephemeral active regions. Network fields are frequently observed to split into smaller fragments and equally often, small fragments are observed to merge or coalesce into larger clumps; this splitting and merging is generally confined to the borders and vertices of the convection cells known as supergranules. Intranetwork magnetic fields originate near the centers of the supergranule convection cells and appear to increase in magnetic flux as they flow in approximate radial patterns towards the boundaries of the cells.

Global Evolution of Photospheric Magnetic Fields

The main features concerning the evolution of the large scale photospheric magnetic fields derived from synoptic maps as well as from H-alpha synoptic charts are reviewed. The significance of a variety of observations that indicate the presence of a high latitude component as a counterpart to the sunspot phenomenon at lower latitudes is reviewed. It is argued that these two components describe the global magnetic field on the sun. It is demonstrated that this scenario is able to link many phenomena observed on the sun (coronal emission, ephemeral active regions, geomagnetic activity, torsional oscillations, polar faculae and global modes in the magnetic field pattern) with the global magnetic activity.

Seventy-five hours of coordinated videomagnetograph observations

view Abstract Citations (35) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Seventy-Five Hours of Coordinated Videomagnetograph Observations Wang, Haimin ; Zirin, Harold ; Patterson, Alan ; Al, Guoxiang ; Zhang, Hongqi Abstract Videomagnetograph observations obtained between September 24 and 29, 1987 are presented which illustrate the evolution of magnetic flux surrounding a stable sunspot. It is found that the dominant sunspot mainly ejects magnetic fields of opposite sign, and that the surrounding plage fields steadily contract and retreat inward toward the umbra, resulting in shrinking and weakening of the spot and plage. The extent of the moat is shown to be reduced by 50 percent in a 75-hour period, with the principal loss of flux probably due to concellation at the main neutral line. Five subflares were noted, three occurring prior to cancellation of the magnetic elements at the inversion line and two occurring during the development and disappearance of an ephemeral bipolar region. Publication: The Astrophysical Journal Pub Date: August 1989 DOI: 10.1086/167722 Bibcode: 1989ApJ...343..489W Keywords: H Alpha Line; Solar Activity; Solar Magnetic Field; Solar Observatories; Sunspot Cycle; Cross Correlation; Magnetic Signatures; Solar Flux; Solar Physics; SUN: MAGNETIC FIELDS full text sources ADS |

The extended solar activity cycle

The solar cycle has been defined in terms of a sequential periodic variation in sunspot numbers, the period being the interval between successive minima, currently averaging 11.2 years. But a number of observations have indicated that the activity cycle may begin at higher latitudes before the emergence of the first sunspots of the new cycle. Here we report results from sunspot cycle 21 concerning the ephemeral active regions, the coronal green-line emission and the torsional oscillation signal, which confirm the earlier suggestions. In particular, we report the appearance of a high-latitude population of ephemeral active regions in the declin-ing phase of sunspot cycle 21, with orientations that tend to favour those for cycle 22 rather than 21. Taken together, these data indicate that sunspot activity is simply the main phase of a more extended cycle that begins at high latitudes before the maximum of a given sunspot cycle and progresses towards the equator during the next 18–22 yr, merging with the conventional 'butterfly diagram' (the plot of the latitudes of emerging sunspots against time) as it enters sunspot latitudes. We suggest that this extended cycle may be understood in the perspective of a model of giant convective rolls that generate dynamo waves propagating from pole to equator.

Microwave structure of the quiet sun

view Abstract Citations (30) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Microwave Structure of the Quiet Sun Gary, Dale E. ; Zirin, Harold Abstract The authors present two-frequency VLA observations of the quiet Sun obtained on 1985 July 13, when the Sun was particularly quiet. The 4.9 GHz full-day synthesis maps correspond closely to images made in Hα, Ca II K, and magnetograms, while the correspondence is much reduced at 1.45 GHz. The 4.9 GHz maps have a meshlike appearance reminiscent of chromospheric network structure. Simultaneous magnetograms from Big Bear Solar Observatory are used to compare the structure of the magnetic field with the radio source structure at the two frequencies. Magnetograms of each pointing field were taken at least once every half-hour for monitoring the changing structure of the longitudinal component of the magnetic fields. The authors identify changes such as the appearance of an ephemeral region and the cancellation of opposite-polarity magnetic flux, and discuss corresponding changes in radio sources. Publication: The Astrophysical Journal Pub Date: June 1988 DOI: 10.1086/166443 Bibcode: 1988ApJ...329..991G Keywords: Chromosphere; Magnetic Signatures; Solar Magnetic Field; Solar Radio Emission; Very Large Array (Vla); Astronomical Maps; Electron Acceleration; Radio Astronomy; Solar Physics; SUN: CHROMOSPHERE; SUN: MAGNETIC FIELDS; SUN: RADIO RADIATION full text sources ADS |

Dynamical oscillation and propulsion of magnetic fields in the convective zone of a star. VI - Small flux bundles, network fields, and ephemeral active regions

view Abstract Citations (11) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical oscillation and propulsion of magnetic fields in the convective zone of a star. VI - Small flux bundles, network fields, and ephemeral active regions Parker, E. N. Abstract Simple analysis of the hydrodynamics initiated by the presence of a coherent flux bundle in the lower half of the convective zone of a star like the Sun indicates that bundles with diameters comparable to the local pressure scale height (5 x 10 cm) are subject to sidewise motions perhaps as fast as 10 cm 51 The motions arise as a consequence of the bundle being squeezed between the cool shadow above and the accumulating heat below, and because of convective propulsion. This suggests that the equatorward motion (102 cm s1) of the azimuthal fields, inferred from observations of the Sun, may be as much a consequence of the hydrodynamics of flux bundles and meridional circulation as it is of the dynamo effects that create the flux. Subject headings: hydromagnetics - stars: interiors stars: magnetic Publication: The Astrophysical Journal Pub Date: March 1988 DOI: 10.1086/166103 Bibcode: 1988ApJ...326..407P full text sources ADS |

The identification and interaction of network, intranetwork, and ephemeral-region magnetic fields

Network magnetic fields, ephemeral active regions, and intranetwork magnetic fields are illustrated and discussed in several contexts. First, they are presented in relation to the appearance and disappearance of magnetic flux. Second, their properties in common with all solar magnetic features are discussed. Third, their distinguishing characteristics are emphasized. Lastly, their interactions are illustrated. Network magnetic fields are no longer considered to be just the aged remnants of active regions. The network is the dynamic product of the merging and cancelling of intranetwork fields, ephemeral regions, and the remnants of active regions. Intranetwork fields are magnetic fields of mixed polarity that appear to originate continuously from localized source sites in between the network. The intranetwork magnetic fields are characterized by flow of successive fragments in approximately radial patterns away from their apparent source sites and by the relative weakness of their magnetic fields. Ephemeral active regions are small, new bipoles that grow as a unit or a succession of bipolar units and whose poles move in opposite directions from their apparent site of origin. Large ephemeral regions are not distinguishable from small active regions.

Coronal bright points at 6cm wavelength

Results are presented from observations of bright points at a wavelength of 6-cm using the VLA with a spatial resolution of 1.2 arcsec. During two hours of observations, 44 sources were detected with brightness temperatures between 2000 and 30,000 K. Of these sources, 27 are associated with weak dark He 10830 A features at distances less than 40 arcsecs. Consideration is given to variations in the source parameters and the relationship between ephemeral regions and bright points.

Long-term variability of solar magnetic fields

The solar magnetic field varies on all time scales. Recent analysis of 600 million year-old Australian varves shows terrestrial evidence for not only the familiar 22-year magnetic cycle, but variations with periods of 300–400 years as well. The Maunder minimum is but one of several long intervals showing atypical levels of solar activity. Recently a 151-day periodicity in flare activity has been found. Active regions, sunspots, ephemeral regions, and flares cover a broad range of shorter time scale variations in the solar magnetic field. Long-term variations can be interpreted in at least two ways. One outlook regards the large-scale and long-term variations of the photospheric field as more-or-less direct guides to the organization of the solar field as it evolves through a solar cycle. The slowly varying field reveals the fundamental interior structure of the Sun. An alternative view interprets the surface manifestation of the magnetic field primarily as the result of the convective motions in the solar atmosphere. The evolving distribution of photospheric flux depends upon the locations of emerging flux and the subsequent motions and interactions of the fields in the moving plasma. Information about the interior field comes largely from analysis of emerging flux. We discuss interpretation of the phenomenology of the long-term variability in the context of these contrasting views.

On the relationship between magnetic fields and supergranule velocity fields

We studied the size, correlation lifetime and horizontal velocity amplitude of supergranules in regions with different magnetic activity. We found that the supergranule velocity cells have similar scale, correlation lifetime and horizontal velocity amplitude in the unipolar enhanced magnetic network regions and in the mixed-polarity quiet Sun. However, the correlation lifetime of magnetic structure is much longer in the enhanced network. We investigated the velocity pattern of moving magnetic features (MMF) surrounding a decaying sunspot. The velocity of MMFs is consistent with the outflow surrounding the sunspot as measured by Dopplergrams. The velocity cell surrounding the sunspot has a much larger velocity amplitude and a longer lifetime than regular supergranule cells. We found that ephemeral regions (ER) have a slight tendency to emerge at or near boundaries of supergranules. Almost all the magnetic flux disappears at the supergranule boundaries. In most cases, two poles of cancelling features with opposite magnetic polarities approach along the boundaries of supergranules.

Structure of magnetic fields on the quiet sun

To obtain quantitative temporal and spatial information on the network magnetic fields, we applied auto- and cross-correlation techniques to the Big Bear videomagnetogram (VMG) data. The average size of the network magnetic elements derived from the auto-correlation curve is about 5700 km. The distance between the primary and secondary peak in the auto-correlation curve is about 17000 km, which is half of the size of the supergranule as determined from the velocity map. The correlation time is about 10 to 20 hours. The diffusion constant derived from the cross-correlation curve is 150 km2 s-1. We also found that in the quiet regions the total magnetic flux in a window 3′ × 4′ changes very little in nearly 10 hours. That means the emergence and the disappearance of magnetic flux are in balance. The cancelling features and the emergence of ephemeral regions are the major sources for the loss and replenishment of magnetic flux on the quiet Sun.

Variation of magnetic fields and flows during the solar cycle

We have studied dynamo models with realistic assumptions for the induction effects and compare the resulting magnetic fields with observations. We make use of recent results of helioseismology to infer the differential rotation and adopt mixing length approach to get the form of the α-effect, which is in general anisotropic. Dynamo models are computed numerically using an eigenvalue method. Finally, we discuss the resulting Lorentz force, which is relevant for explaining observed flows such as the torsional waves, as a back reaction of the dynamo magnetic fields. Comparison is made in particular with Mt. Wilson/Kitt Peak magnetograms and synoptic charts showing the magnetic flux, torsional oscillation pattern, and the distribution of ephemeral regions, coronal activity, and polar faculae.

Evolution of network magnetic fields of solar quiet regions

There are 4 types of evolution patterns of network magnetic fields: (1) flux cancellation, the mutual disappearance of encountering fluxes of opposite polarity, (2) flux increase by emergence of ephemeral regions, (3) flux decrease of one polarity and (4) flux increase of one polarity, without emergence of ephemeral regions. From a time sequence of magnetograms of a quiet region of 1983 October 14, the evolution of 300 network features was measured. The magnetograms have a spatial resolution of 2 to 3 arcsec and a time resolution of about 2 hr. The statistics show that the contribution to flux decrease by Type 3 is 1.28 times that by Type 1, and the contribution to flux increase by Type 4 is 7 times that by Type 2.

Weak solar fields and their connection to the solar cycle

Videomagnetograph observations of the weak solar magnetic fields are discussed. Intranetwork (IN) elements are found to emerge in bipolar form, but to flow in a random pattern rather than toward the network elements. Merging of IN fields is found to be more important than the ephemeral regions as a source of new network elements. Arguments for and against the hypothesis that all solar fields are intrinsically strong are considered. It is noted that reconnection and merging of magnetic fields take place continually under the conditions presently examined.

On the maintenance of sunspots - an ion hurricane mechanism

Inward and downflow motions below the photosphere are considered to be a means of cooling and powering sunspot dynamics. Parker's superadiabatic effect is examined with attention focused on the energetics involved in ionization advection. The current analysis enhances Parker's mechanism by allowing for a 2 m/s downflow velocity at 2000 km depth to significantly reduce the photospheric irradiance to near umbral intensities. Similarities between sunspots and terrestrial hurricanes are noted, and it was found that ionization energy plays the same role in sunspots as latent energy plays in terrestrial weather systems. Based on the hypothesis that magnetic fields are important in organizing the motions on the sun, some understanding is provided of: (1) the instability which drives the cooling mechanism for sunspots, (2) the low latitude appearance of sunspots, (3) active region development with faculae following sunspot growth, (4) the role of the fluid in maintaining the magnetic field and the role of the field as a focal point for the fluid downflow, (5) the heating mechanism and structure of faculae, (6) energy balance in active regions, and (7) the behavior difference of pores and ephemeral active regions in relation to ordinary sunspots and active regions.

The smallest observable elements of magnetic flux

We have followed disappearing elements of magnetic flux to determine the smallest elements detectable with the Big Bear videomagnetograph. All the elements followed were disappearing through interaction with elements of opposite polarity. The last remaining visible segment of magnectic field of such features can be used to infer the total magnetic flux of these and other small flux elements visible on the magnetograms. We used both photographic and digital videomagnetograms combining 4096 Zeeman frames made at Big Bear. Fifteen elements were measured near the vanishing point, in a 2–8 hr period. The minimum observable fluxes fall in the range of 1.0 × 1016 to 1.4 × 1017 Mx, and the apparent size of these elements is in the range of 1 to 9 square arc sec. The process of disappearance appears to be a smooth one. The smallest detectable elements of network field and ephemeral regions (ER) appear to be the same as the small intra-network (IN) field elements. The present limit is still instrumental; elements smaller than 1 × 1016 would not have been detected.